Charge pump circuits are widely used by electronic designers. Charge pumps may be configured as both positive and negative systems. Given a positive reference voltage (for example, a bandgap reference voltage), negative charge pump systems are generally required to level shift the negative output voltage to a positive feedback voltage for comparison with the positive reference voltage for closed loop feedback regulation.
A negative pump system typically comprises a series of charge pump cells. The output of the pump is level shifted and compared with a reference voltage to generate an enable signal for the pump clock drive system. The pump clock drive signal drives the charge pump cells.
In the conventional art using a positive reference voltage, the output of the pump is shifted to a positive voltage so that it can be compared with the (positive) reference voltage in order to control regulation. Conventional art circuits use, for example, resistor divider stacks wherein the highest potential voltage is the positive reference voltage and the negative potential is used as the negative feedback voltage. A supply voltage generally cannot be used as the highest potential in such a resistor divider stack as the negative pump output voltage would then vary with variations in supply voltage. Such variations are especially unsuitable for applications operating under wide supply voltage variations.
Hence, a positive reference voltage is used as the highest potential in the resistor divider stacks of conventional art negative pump systems. Using this technique, a designer can tap a positive level shifted version of the negative feedback voltage that can then be compared with a positive reference in the closed loop system. For such a system to have a desirably fast response to supply voltage variations, the resistor stack should have a low resistance. For example, the reference voltage serving as the highest positive potential in the resistor divider stack must be able to supply current. Unfortunately, typical voltage references are neither designed nor capable of supplying such current loads. For example, such typical voltage references are generally high impedance sources.
To overcome such shortcomings, conventional art systems typically use an operational amplifier buffer in the reference voltage path in order to provide such current loads. Utilizing an operational amplifier in such a manner generally requires an operational amplifier with very fast response characteristics in order for the charge pump feedback system to have desirably fast response and turn-on characteristics.